Assalamualaikum ustaz. What is the status of the syahadah for a mute? Is his Islam status valid when his syahadah is just by sign language? Hope for the explanation. Thank you.
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.
Syahadah (شهادة) lexically is a part of the word (شهد) which means bear witness.  the word syahadah can also mean telling something that one witnesses, interpreting what one already knows. 
Reciting the two syahadah is the first pillar of the five pillars of Islam. This is as stated in a hadith from Ibnu Umar R.Anhuma, where Rasullullah PBUH said:
بُنِيَ الْإِسْلَامُ عَلَى خَمْسٍ شَهَادَةِ أَنْ لَا إِلَهَ إِلَّا اللَّهُ وَأَنَّ مُحَمَّدًا رَسُولُ اللَّهِ وَإِقَامِ الصَّلَاةِ وَإِيتَاءِ الزَّكَاةِ وَالْحَجِّ وَصَوْمِ رَمَضَانَ
“Islam is based on five things: the testimony that there is no god but God and that Muhammad is His servant and messenger, the observance of the prayer, the payment of zakat, the Pilgrimage, and the fast during Ramadan.” 
In the hadith narrated from ‘Ubadah R.A, Rasullullah PBUH said:
مَنْ شَهِدَ أَنْ لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ وَأَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ وَأَنَّ عِيسَى عَبْدُ اللَّهِ وَرَسُولُهُ وَكَلِمَتُهُ أَلْقَاهَا إِلَى مَرْيَمَ وَرُوحٌ مِنْهُ وَالْجَنَّةُ حَقٌّ وَالنَّارُ حَقٌّ أَدْخَلَهُ اللَّهُ الْجَنَّةَ عَلَى مَا كَانَ مِنْ الْعَمَلِ
“If anyone testifies that there is no god but God alone, who has no partner, that Muhammad is His servant and messenger, that Jesus is God’s servant and messenger, the son of His handmaid, His word which He cast into Mary and a spirit from Him, and that paradise and hell are real, God will cause him to enter paradise no matter what he has done.” 
The status of the syahadah of a mute
Allah SWT doesn’t burden His slave more than what he is capable. This is based on His statement:
لَا يُكَلِّفُ اللَّـهُ نَفْسًا إِلَّا وُسْعَهَا
“Allah does not charge a soul except [with that within] its capacity.” 
Syeikh al-Maraghi said: Allah SWT doesn’t burden His slaves with something more than their capabilities. Rather, His blessing eases and provide facilitation for them. 
In the above issue, we present here the opinion of Imam al-Nawawi who among others said: “The Muslim status of a mute who recites the syahadah through understandable sign language is valid. However, in another opinion, it is stated that his Muslim status isn’t acknowledged until after he prayed after reciting his syahadah in sign language. This is the literal opinion of Imam al-Syafi’I in his book, Al-Umm. The true and ma’ruf opinion is the first opinion. Whereas the opinion of Imam Syafi-I should be understood in the context when (the syahadah) in sign language isn’t understood.” 
A maxim stated:
الإِشَارَةُ المعْهُوْدَةُ لِلْأَخْرَسِ، كَالْبَيَانِ بِاللِّسَانِ
“Sign language that is understood by a mute is the same as a verbal statement.” 
Syeikh Ahmad al-Zarqa’ explained: Any sign that is understood made by a mute used usually by a mute using his body parts such as his hands or head is considered as verbal speech or explanation. 
According to the above statement, in our opinion, a mute who embraces Islam using an understood sign language is valid.
May Allah SWT grant us all knowledge and the correct understanding in fulfilling His every commandment on this earth. Amin.
 See al-Munjid fi al-Lughah, pg. 406
 See Mu’jam al-Wasit, pg. 516
 Narrated by al-Bukhari (8)
 Narrated by Bukhari (3435)
 Surah al-Baqarah: 286
 See Tafsir al-Maraghi, 2/672
 See Raudhah al-Talibin, 7/282
 See al-Asybah wa al-Naza’ir by al-Suyuti, pg. 31; al-Asybah by Ibn Nujaim, pg. 343 and al-Wajiz, pg. 244
 See Syarh al-Qawa’id al-Fiqhiyyah, pg. 351