Muwaththa' Malik · #453
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يَقُولُ " إِذَا أَنْشَأَتْ بَحْرِيَّةً ثُمَّ تَشَاءَمَتْ فَتِلْكَ عَيْنٌ غُدَيْقَةٌ " .
English
Yahya related to me from Malik that he had heard that the Messenger of Allah, may Allah bless him and grant him peace, 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