It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "The gates of heaven are opened and the supplication of a Muslim is answered: when the prayer is established (iqamah), when the rain falls, when the ranks advance, and on seeing the Ka'bah."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7688