Marwan ibn Salim narrates, I saw Ibn Umar take hold of his beard and cut off what exceeded a fist, and he (Ibn Umar) said that the Messenger of Allah peace be upon him would recite the following when he would open his fast,

The thirst has gone, the veins have been moistened and the reward has been fixed if Allah wills.
Note: This Dua is to be recited after opening the fast as mentioned by numerous commentators on this narration.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدِ بْنِ يَحْيَى أَبُو مُحَمَّدٍ حَدَّثَنَا عَلِىُّ بْنُ الْحَسَنِ أَخْبَرَنِى الْحُسَيْنُ بْنُ وَاقِدٍ حَدَّثَنَا مَرْوَانُ – يَعْنِى ابْنَ سَالِمٍ – الْمُقَفَّعُ –
قَالَ رَأَيْتُ ابْنَ عُمَرَ يَقْبِضُ عَلَى لِحْيَتِهِ فَيَقْطَعُ مَا زَادَ عَلَى الْكَفِّ وَقَالَ كَانَ رَسُولُ اللَّهِ -صلى الله عليه وسلم- إِذَا أَفْطَرَ قَالَ

ذَهَبَ الظَّمَأُ وَابْتَلَّتِ الْعُرُوقُ وَثَبَتَ الأَجْرُ إِنْ شَاءَ اللَّهُ .

(رواه أبو داود،باب الْقَوْلِ عِنْدَ الإِفْطَارِ، ح/٢٣٥٩)