Supplication That Will Not Be Rejected...

Ibn Majah related from 'Abdullah ibn 'Amr ibn al-'Aas that the Prophet, sallAllaahu 'alayhi wa sallam, said: 

"A fasting person, upon breaking his fast, has a supplication that will not be rejected." 

When 'Abdullah broke his fast he would say: "O Allah, I ask of You, by Your mercy that encompasses everything, to forgive me."