Is the supplication at Safa recited upon ascent or before it, and is it repeated if it ends before reaching the top?

It is sunnah to recite the remembrance (dhikr) reported from the Prophet, peace and blessings be upon him, after reaching Safa and standing upon it, whether one ascends to its summit or stands below it in an area considered part of it. This dhikr should be repeated three times, then one should supplicate (du'a) and also repeat it three times. It is likewise prescribed to repeat the dhikr and du'a at Marwah. The dhikr is: "There is no god but Allah alone, He has no partner. To Him belongs sovereignty, and to Him belongs all praise, and He is over all things competent. There is no god but Allah alone; He fulfilled His promise, granted victory to His servant, and defeated the confederates alone."