The Sunnah preserves the words the Prophet ﷺ said when he broke his fast, his du'a for the one who feeds you, and the du'a he taught 'A'ishah for Laylat al-Qadr.
Dhahaba-dh-dhama'u, wabtallati-l-'uruqu, wa thabata-l-ajru in sha' Allah.
“The thirst has gone, the veins are moistened, and the reward is certain, if Allah wills.”
Ibn 'Umar reported that the Prophet ﷺ said this when he broke his fast.
Allahumma at'im man at'amani, wa asqi man asqani.
“O Allah, feed the one who fed me, and give drink to the one who gave me drink.”
Al-Miqdad reported that the Prophet ﷺ raised his head to the sky and said this on a night his share of milk had been drunk. It suits anyone who hosts you, including the one who gives you iftar.
Allahumma innaka 'Afuwwun tuhibbu-l-'afwa fa'fu 'anni.
“O Allah, You are the Pardoner and You love to pardon, so pardon me.”
'A'ishah asked the Prophet ﷺ what she should say if she knew which night was Laylat al-Qadr, and he taught her these words. Some printed editions of at-Tirmidhi add "karim" (Generous) after "'Afuwwun"; the wording in Ibn Majah, given here, does not have it.
Aftara 'indakumus-sa'imun, wa akala ta'amakumul-abrar, wa sallat 'alaykumul-mala'ikah.
“May those who fast break their fast with you, may the righteous eat your food, and may the angels pray for you.”
Said to the hosts after eating at their home, especially at iftar.
Inni sa'im, inni sa'im.
“I am fasting, I am fasting.”
Said calmly to anyone who insults or quarrels with you while you are fasting.