Etiquette at time of saheri and iftar

'One of the most important etiquette and instruction is the recital of Surah al-Qadr (Power) before taking saheri and iftar as well as recital of illustrious and illuminated supplication:'

Allahuma rabb al-nur al-'adheem

Allah is the possessor of great illumination

Source:Spiritual Journey of the Mystics (Suluk-i Arifan), Etiquette of the Holy Month of Ramadhan, A comprehensive Commentary About the Esoteric Essence of the Holy Month of Ramadhan by Haj Mirza Javad Agha Maliki Tabrizi (r.a.)

Supplication at time of breaking the fast:

Allaahumma laka sumtu wa 'alaa riz-qika aftartu wa 'alayka tawakkaltu

O Allah: For the sake of You have I fasted;
And with Your sustenance have I broken my fast;
And in You have I trusted (relied).

Source: The Rites of Rajab, Sha'ban, Ramadan: Supplications, Prayers, and Ziyararahs

Bismillah allaahumma laka sumnaa wa 'alaa riz-qika aftarnaa fa-taqabbal minnaa innaka antas samee-ul 'aleem

In the name of Allah, O Allah, we fast, and with the food Thou gives us we break the fast, an obligation we fulfill, and Thou are Hearer, Knower

 

Bismillahir rah'maanir rah'eem yaa waasi-al maghfira igfir lee

In the name of Allah, the Beneficent, the Merciful, O He whose indulgence is boundless, forgive me

Source: Biharul Anwar, Volume 51, Page 147; al-Ghaybah of al-Num'mani