Musannaf Ibn Abi Shayba 21:122

Ma'n narrated to us, from Malik ibn Anas, from Abu Hazim, from Sahl ibn Sa'd al-Sa'idi, who said: There are two times during which the gates of the heaven are opened, and rarely is the supplication of a supplicant rejected during them: at the time of the call to prayer, and when the rows are formed in the path of Allah, the Mighty and Majestic.