Dhikr and dua at Safa and Marwah
The authentic remembrance said while facing the qiblah at Safa and again at Marwah. The Prophet ﷺ praised Allah (SWT) with these words, made his own dua between repetitions, and repeated the sequence three times.
At Safa, after facing the qiblah, say Allah (SWT)u Akbar and this remembrance, then make your own dua. Repeat the remembrance-and-dua sequence three times. Do the same at Marwah. There is no authentic different dua assigned to each lap of sa'i.
The Arabic and meaning are paired with Sahih Muslim 1218a. No special promise or count is added without a cited basis.
La ilaha illa-llahu wahdahu la sharika lah, lahu-l-mulku wa lahu-l-hamd, wa Huwa 'ala kulli shay'in Qadir. La ilaha illa-llahu wahdah, anjaza wa'dah, wa nasara 'abdah, wa hazama-l-ahzaba wahdah.
There is no deity except Allah (SWT) alone, with no partner. His is the dominion and His is the praise, and He has power over everything. There is no deity except Allah (SWT) alone. He fulfilled His promise, helped His servant, and defeated the confederates alone.
Jabir (RA)'s detailed report of the Prophet ﷺ's ﷺ Hajj explains the practice at Safa and Marwah. After reciting that Safa and Marwah are among Allah (SWT)'s signs, the Prophet ﷺ faced the qiblah, declared Allah (SWT)'s oneness, praised Him with these words, and then supplicated for what he wished.
The narration leaves the personal supplication open. This is more useful than memorising invented lap-by-lap formulas: begin with the transmitted remembrance, then ask Allah (SWT) sincerely for your own needs.
Is there a different dua for every round of sa'i?
No authentic narration assigns a separate dua to each round. At Safa and Marwah, use the transmitted remembrance and make your own supplications between its repetitions.
Do I say this at both Safa and Marwah?
Yes. Sahih Muslim 1218a reports that the Prophet ﷺ did at Marwah as he had done at Safa.
What can I ask Allah (SWT) for between the repetitions?
The narration says the Prophet ﷺ made supplication without restricting it to one formula, so ask for your needs in this life and the Hereafter.