It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (ﷺ) said: "When you see the moon, then fast, and when you see the moon, then break the fast. If the sky is cloudy, then complete the count to thirty." The narrator says that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) would fast one day before seeing the moon. Ibrahim (may Allah have mercy on him) was asked if he also fasted one day before, and he said, "Yes."
Hadith Referenceالسنن المأثورة / باب ما جاء في تقدم الشهر / 335
Hadith Takhrijسنن ابن ماجه ، الصيام، باب ماجاء في صومو الرؤتيه ......، رقم : 1654 وقال الالبانی: صحیح، مسند احمد : 402/10، رقم : 6323 وقال الارنوؤط: اسناده صحيح