Sahl bin Sa'd (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "There are two supplications that are not rejected", or he (peace and blessings be upon him) said: "that are very rarely rejected: one at the time of the call (to prayer), and the other at the time of battle, when they are cutting one another down." Sahl bin Sa'd (may Allah be pleased with him) also narrates from him (peace and blessings be upon him) that he (peace and blessings be upon him) said: "And under the rain."
This has also been narrated from Abu Umamah (may Allah be pleased with him) from the Prophet (peace and blessings be upon him), except that Ufayr bin Ma'dan is in its chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6459
Sayyiduna Abu Umamah (may Allah be pleased with him) was heard narrating from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "The gates of the sky are opened and supplication is answered on four occasions: 1. when the ranks meet (in battle), 2. when the rain comes down, 3. when the prayer is established (iqamah), 4. when the Ka'bah is seen."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6460