Mujahid (may Allah have mercy on him) said: I was sitting with Ibn Abbas (may Allah be pleased with them both) when a man came to him and said: O Abu Abbas, I have divorced my wife three times. Ibn Abbas (may Allah be pleased with them both) said: "You have disobeyed your Lord, and your wife has become separated from you. Allah, the Mighty and Majestic, says: 'O Prophet, when you divorce women, divorce them at (the start of) their waiting period (iddah)'" [Al-Talaq: 1].
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11093