Muwatta Malik 453

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, 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

Reference

Muwatta Malik 453

In-book

Book 13, Hadith 5