بسم الله الرحمن الرحيم
QUESTION: Assalamualaikum Wr wb. Is it obligatory to reply salams that were sent by sms or whatsapp?

ANSWER: Wa alaikummus Salam Wr wb:
It is compulsory to reply salam regardless whether it comes in a way of speech or writing. Allah s.w.t said in the Quran Surah An-nisa verse 86:
وَإِذَا حُيِّيتُمْ بِتَحِيَّةٍ فَحَيُّوا بِأَحْسَنَ مِنْهَا أَوْ رُدُّوهَا
When you are greeted with salams, reply it in a better way or like it.
As for the explanations of replying different types of salams,
Ibn Hajar Al-Haitami mentuioned in his book Al-Fatawa Al-Fiqhiyyah as follows:
 
وأما المرسل إليه: فلزمه الرد فورًا، ثم إن كان السلام عليه بالإرسال لزمه الرد باللفظ، وإن كان بالكتابة لزمه الرد بها، أو باللفظ
As for the one whom the salam is directed by a messenger: Replying to it spontaneously is compulsory. This is done by saying it out if it comes by way of speech. If it reaches him by way of writing, then it is compulsory upon him to reply the salam by writing or by just by saying.
، ويندب الرد على الرسول أيضًا، وتقديمه، فيقول: وعليك وعليه السلام، وكأن سبب عدم جعلهم قوله، وعليك السلام قاطعًا لفورية الرد؛ لأنه غير أجنبي، فكما اغتفروه في عدم قطعه لفورية القبول في نحو البيع، فكذلك يغتفر الفصل به هنا،
It is recommended upon him to address the messenger too in the reply by mentioning him first by saying alaika wa alaihi salam- peace be upon you and him. The reason the ulama do not consider this form of address as something that breaks the fawriyyah (continuity) as it not considered to be a non-relating phrase. Just like how they considered it as something that is excused due to its validity of the spontaneous response despite its interference like the case of buying and selling. Likewise it is permissible here.
Till he said….
………….وأما وجوب الرد بالكتابة: فحكمته ظاهرة؛ لأن الكتاب إذا وصل للمسلم كان بمنزلة الرد عليه حينئذ
As for the compulsion of replying salam by writing, the wisdom in it is prevalent. This is because the salam that reaches him in the form of writing is considered as a valid form of reply.
الفتاوي الفقهية (4/243
[Al-Fatawa-Al Fiqhiyyah vol 4 pg 243]
 
Thus replying salam in the form of writing is compulsory likewise in the form of speech.
Wallahu A’lam

Contributor

Ustaz Anwar Hussain