Short Answer

Yes, it is fine to write out abbreviations since it is understood by the reader what it means. Also, this is not disrespectful since writing is often slower than speaking, so there is good justification for abbreviations.

What the Scholars Said

Imam Nawawi writes, “It is slightly recommended (mustahabb) for a person writing Hadith that…..at the mentioning of the Messenger of Allah, he writes “sallallahu alayhi wasallam” (peace be on him) in full and not merely using abbreviations, and neither sufficing on one of the two, i.e. salah & salam.” (Sharh Sahih Muslim, 1:39)

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Shaykh Mustafa Umar