The Quran

وَأَنكِحُوا الأَيامىٰ مِنكُم وَالصّالِحينَ مِن عِبادِكُم وَإِمائِكُم ۚ إِن يَكونوا فُقَراءَ يُغنِهِمُ اللَّهُ مِن فَضلِهِ ۗ وَاللَّهُ واسِعٌ عَليمٌ

Muhammad Asad

AND [you ought to] marry the single from among you¹ as well as such of your male and female slaves as are fit [for marriage].² If they [whom you intend to marry] are poor, [let this not deter you;] God will grant them sufficiency out of His bounty - for God is infinite [in His mercy], all-knowing.

I.e., from among the free members of the community, as is evident from the subsequent juxtaposition with slaves. (As most of the classical commentators point out, this is not an injunction but a recommendation to the community as a whole: hence my interpolation of the words "you ought to".) The term ayyim of which ayama is the plural - signifies a person of either sex who has no spouse, irrespective of whether he or she has never been married or is divorced or widowed. Thus, the above verse expresses the idea - reiterated in many authentic sayings of the Prophet - that, from both the ethical and the social points of view, the married state is infinitely preferable to celibacy.
The term as-salihin connotes here both moral and physical fitness for marriage: i.e., the attainment of bodily and mental maturity as well as mutual affection between the man and the woman concerned. As in 4:25, the above verse rules out all forms of concubinage and postulates marriage as the only basis of lawful sexual relations between a man and his female slave.