Sahih Muslim Book 4, Hadith Number 747.

Chapter : The Holy Prophet refrained from attacking people living in Dar al-Kufr on bearing Adhan from them.

‘Abdullah b. Amr b. al-As reported Allah’s Messenger (may peace be upon him) as saying: When you hear the Mu’adhdhin, repeat what he says, then invoke a blessing on me, for everyone who invokes a blessing on me will receive ten blessings from Allah; then beg from Allah al-Wasila for me, which is a rank in Paradise fitting for only one of Allah’s servants, and I hope that I may be that one. If anyone who asks that I be given the Wasila, he will be assured of my intercession.