From Sahih Bukhari (Hadith 306):
Narrated `Aisha:Fatima bint Abi Hubaish said to Allah's Messenger (ﷺ), "O Allah's Messenger (ﷺ)! I do not become clean (from bleeding). Shall I give up my prayers?" Allah's Messenger (ﷺ) replied: "No, because it is from a blood vessel and not the menses. So when the real menses begins give up your prayers and when it (the period) has finished wash the blood off your body (take a bath) and offer your prayers
Reference: Bukhari 306