It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The gates of heaven are opened and supplication is answered on four occasions: when the ranks meet in the path of Allah, when the rain falls, when the prayer is established (iqamah), and on seeing the Ka'bah."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7682