It is narrated from Sayyiduna Taliq that the Prophet (peace be upon him) said: Allah has made the moon a means for people to determine times, so when you see the moon, observe the fast, and when you see the moon, celebrate Eid. If it is cloudy, then complete the count to thirty.
Hadith Referenceمسند احمد / مسند المدنيين رضي الله عنهم اجمعين / 16294
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد ضعيف لضعف محمد بن جابر