It has been related that on the day of the Battle of the Camel a bedouin came before the Commander of the Faithful and said, "O Commander of the Faithful! Sayest thou that God is one?"
The people attacked him and said, "O bedouin! Do you not see how the Commander of the Faithful's heart is divided (with cares)?"
The Commander of the Faithful said, "Leave him, for surely what the bedouin wishes (i.e. knowledge of God) is what we wish for the people."
Then he said, "O bedouin! To say that God is one (wahid) has four (possible) meanings, two of which are not permissible concerning God, the Mighty and Majestic, and two of which are established concerning Him."
"As for the two which are not permissible concerning Him, (the first is) the saying of him who says 'one' and has in mind the category of numbers. Now this is not permissible, for that which has no second does not enter into the category of numbers. Hast thou not seen that he who says that He is 'the third of three' [This is a reference to the Quranic verse, "They are unbelievers who say, 'God is the Third of Three' " (V, 73).] is of the unbelievers?
And (the second is like) the saying of him who says (concerning a man), 'He is one of mankind', meaning that he is one kind within the species. [The first kind of "unity" that is rejected is numerical unity, or the idea that when we say "He is one God", we mean something similar to what we mean when we say, 'This is one walnut", i.e., that there may also be two Gods, three Gods, etc. The second "unity" refers to similarity in kind or species, as when we say, "this is one cat", meaning that there are also other kinds of cats. In the words of Majlisi, "When it is said in this sense of a Byzantine that he is one of mankind, it is meant that his kind is one of the kinds of men or is a kind among other kinds". This is tashbih because we are comparing God to whatever we say He is one of. Since He is one of that kind, He has to be similar to others of that kind.] This is not permissible because it is a comparison, and our Lord is greater than that and high above it."
"As for the two meanings which are established concerning Him, (the first is) the saying of him who says,
'He is one, there is no likeness (shabah) unto Him among things.'
Such is our Lord.
And (the second is) the saying of him who says,
'Surely He, the Mighty and Majestic, is single in meaning (ahadi al-mana), intending by that that He is not divided by existence, the power of reason, or imagination. Such is our Lord, the Mighty and Majestic." Part I, A Shi'ite Anthology, 'Allamah Sayyid Muhammad Husayn Tabataba'i, Translated with Explanatory Notes by William C. Chittick
'O God, carry us in the ships of Thy deliverance, give us to enjoy the pleasure of whispered prayer to Thee, make us drink at the pools of Thy love, let us taste the sweetness of Thy affection and nearness, allow us to struggle in Thee, preoccupy us with obeying Thee, and purify our intentions in devoting works to Thee, for we exist through Thee and belong to Thee, and we have no one to mediate with Thee but Thee!' Imam Sajjad ('A); Sahifa al-Sajjadiyya