مُوَطَّأُ مَالِكٍ
Muwatta Malik 456
Asking for Rain
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يَقُولُ إِذَا أَنْشَأَتْ بَحْرِيَّةً ثُمَّ تَشَاءَمَتْ فَتِلْكَ عَيْنٌ غُدَيْقَةٌ .
Yahya related to me from Malik that he had heard that the Messenger of Allah, (ﷺ), used to say, "When a cloud appears from the direction of the sea and then goes towards ash-Sham, it will be an abundant source of rain."
Arabic/English book reference: Book 13, Hadith 5