wa rahmatul lahi wa barkatuhu
"Invoke Allah or invoke the Most Gracious (Allah) by whatever name you invoke Him (it is the same), for to Him belong the Best Names. And offer your salat (prayer) neither aloud nor in a low voice, but follow a way between."
Qur'an 17:110
Supplicating to Allah and mediating towards Him tawassul by way of His names is a lawful practice. For example, we can say: “O Rahmân, have mercy on me.” “O Ghaffâr, forgive me.” Allah says: “And Allah has the most beautiful of names, so call to Him by them.” As for the practice of repeating a name 100 times after `Asr and other similar practices, there is no basis for this whatsoever.
And Allah knows best.
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http://islamtoday.com/node/1203
Some Hadiths might help too:
Abu Hurairah reported that whenever the Prophet,
, was faced with a serious difficulty, he would raise his head to the sky and supplicate, "Subhan-Allah al-'Azim (glory be to Allah, the Mighty)." And when he implored seriously and strongly, he would say
"Ya Hayyu, Ya Qayyum (O the Living, the Eternal One)." (Tirmidhi)
Anas reported, "Once the Prophet,
, passed by Abu 'Ayyash, Zayd b. as-Samit az-Zarqi, while he was offering prayer and saying,
'Allahumma Inni As-aluka Bianna Lakal Hamdu La Ilaaha Illa Antal Hannanul Mannanu Badiussamawati wal Ardi Ya Dhal Jalali wal Ikram Ya Hayyu Ya Qayyum As-aluka' [O Allah, I call upon You for to You is due all praise, there is no god but You, O the Compassionate One, O the True Benefactor, the Originator of the heavens and the earth, the Lord of Majesty and Honor, O the Living One, O the One Who is self-subsisting and sustains all.'] The Prophet, peace be upon him, said, 'You have called upon Allah by His greatest name. Anyone calling upon Him by this name is heard, and whatever one asks Him for, He grants it'."
(Reported by Ahmad and others. Al-Hakim says that it is sound according to the criterion of Muslim)
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Fiqh-us-Sunnah
