Narrated Sahel Ibn Sa’ad (RA): that the Messenger of Allah (SAW) said: ‘Two will not be rejected, Supplication when the Adhan (call of prayer) is being called, and at the 

time of the rain’.

Al-Hakim 2: 114, and Abu Dawud #2540, ibn Majah

Seek the response to your du’as when the armies meet, and the prayer is called, and when rain falls’ 

reported by Imam al-Shafi’ in al-Umm, al-Sahihah #1469