Tahiyatul masjid whilst azan is being called
In the name of Allah, the most Beneficent, the most Merciful.
It is mustuhub to respond to the azan and doing so holds great reward. It is mentioned in a hadith that the Prophet of Allah Sallallahu Alahi Wasalam said, “Whoever repeats the words of the muadhin with conviction will enter Paradise.” (Sahih Muslim p.167 v.1)
Therefore, it is preferable not to engage in tahiyatul masjid straight away but instead wait for the azan to finish so that you can give the reply to the azan. However, if someone does start his tahiyatul masjid or any other salah whilst the azan is being said, there is no sin in that. (Raddul Muhtar p.70 v.2)
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham.