Assalamualaikum ustaz. When a person prays, he slapped a mosquito and some of the blood stains his sarong. What is the ruling of his prayer? Hope for an explanation.
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
Among the valid conditions of prayer is for one’s clothing, body and place of prayer to be clean and free from najis. This is based on the statement of Allah SWT:
“And your clothing purify,” 
Syeikh Dr Wahbah al-Zuhaili in his commentary stated that the above verse means to take care of one’s clothing from najis and filth. 
Thus, included as a valid condition of prayer for a person to be pure and free from najis on his body, clothing and place of prayer. This is as stated by al-Khatib al-Syarbini: “Fifth condition: One must be pure and clean from unforgivable najis on one’s clothing and body, which means his clothing and body even if it is inside his nose, mouth, eyes or ears. (One must also ensure purity) in terms of the place where he is praying. A prayer is invalid if there is najis in the places stated although when the person is unaware of its existence or doesn’t know that it invalidates prayer.” 
Hence, if there is unforgivable najis on one’s clothing or place of prayer, his prayer is invalid. Likewise, if he is wearing clothing that came into contact with najis, his prayer is invalid. This is as explained by Imam al-Syirazi: “If a person wears a clean clothing and the end of it comes into contact with najis, for instance, a person wears a turban on his head and the end of it fell on some dirt with najis, then his prayer is invalid, for he bears with what is connected to the najis.” 
Imam al-Nawawi in his commentary stated: “What stated by the author is agreed by scholars. Regardless of whether it comes in contact with the najis when he moves to stand, sit, bow and prostrate or when he is not moving. This is our madhhab and there is no khilaf on this matter.” 
Thus, a person’s prayer is considered invalid if there is najis on his body, place of prayer or he bears najis on himself.
Coming back to the above question, it has been agreed by scholars that the blood of any living creature including humans are najis.  Among the evidences which state the najis status of blood is a hadith narrated by Asma binti Abu Bakar R.Anha, where a woman went to meet the Prophet PBUH and asked regarding menstruation blood that smears a garment, the Prophet PBUH said:
تَحُتُّهُ ثُمَّ تَقْرُصُهُ بِالْمَاءِ ثُمَّ تَنْضَحُهُ ثُمَّ تُصَلِّي فِيهِ
“She should scrape it, rub it with water, then wash it and then she may pray in it.” 
However, a question regarding if the amount of blood that smeared on a person is a little, does it invalidate prayer or is it valid to wear clothing where there is a little amount of blood smeared on it?
According to Imam al-Nawawi, when a person kills a mosquito and lice and its blood smeared on his clothing or stains the edges of his fingernails, it is najis if the amount is a lot. If its amount is a little, then there is khilaf among scholars, and the most sahih opinion is that it is forgiven. 
According to al-Khatib al-Syarbini, among the examples of forgiven blood and included under the category of ‘umum al-balwa that is hard to be purified such as the blood of mosquito, fly, lice and other minute amounts of blood that is hard to be seen. 
This opinion is in line with the principle and universal Islamic shariah which is: Islam is an easy and easing (taisir wa raf’ul haraj). Allah SWT states:
يُرِيدُ اللَّهُ بِكُمُ الْيُسْرَ وَلَا يُرِيدُ بِكُمُ الْعُسْرَ
“Allah intends for you ease and does not intend for you hardship,” 
Al-Qurtubi said: Wajh in this verse is a general form of the phrase in any matters related to religion. 
Allah SWT further states:
وَمَا جَعَلَ عَلَيْكُمْ فِي الدِّينِ مِنْ حَرَجٍ
“…and (Allah) has not placed upon you in the religion any difficulty…” 
Al-Sobuni said: “Allah did not make for you in this religion any form of hardship and masyaqqah. He also never set for you, what you can’t bear, moreover, it is a religion filled with tolerance.” 
To conclude, a small amount of blood from the bite of a mosquito, leech and others if smeared on a person who is praying will not invalidate his prayer, for the najis is in a small amount and is included under the matters forgiven by syara’.
The determination of a lot or a little follows the uruf and the blood of mosquito is included under a small amount of blood.