Invocations for breaking the fast
 

ذَهَـبَ الظَّمَـأُ، وَابْتَلَّـتِ العُـروق، وَثَبَـتَ الأجْـرُ إِنْ شـاءَ الله

176. Thahabadh-dhama'u wabtallatil-'urooqu, wa thabatal-'ajru 'inshaa'Allaah.

The thirst is gone, the veins are moistened and the reward is confirmed, if Allah wills.

Reference:
Abu Dawud 2/306 and others. See also Al- Albani, Sahihul-Jami' As-Saghir 4/209.

اللّهُـمَّ  إِنَّـي أَسْـأَلُـكَ بِرَحْمَـتِكَ الّتي وَسِـعَت كُلَّ شيء، أَنْ تَغْـفِرَ لِي

177. Allaahumma 'innee 'as'aluka birahmatikal-latee wasi'at kulla shay'in 'an taghfira lee.

O Allah, I ask You by Your mercy, which encompasses all things, that You forgive me.

Reference:
Ibn Majah 1/557 from a supplication of Abdullah bin 'Amr. Al-Hafidh graded it as good in his checking of An-Nawawi's Kitabul-'Athkdr. See Sharhul- Athkar 4/342.