It is narrated from Sayyiduna Sabrah (may Allah be pleased with him), who reported it from the Prophet (peace and blessings be upon him) as a statement of the Prophet (marfu'), that a cloud arose and the Companions of the Prophet (peace and blessings be upon him) said: We were hoping that this cloud would rain upon us. So he (peace and blessings be upon him) said: "This is a cloud that has been commanded to rain" - meaning upon (that) valley.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6525