Allah's saying:

﴿وَلْيَسْتَعْفِفِ الَّذِينَ لاَ يَجِدُونَ نِكَاحاً حَتَّى يُغْنِيَهُمُ اللَّهُ مِن فَضْلِهِ﴾

(And let those who find not the financial means for marriage keep themselves chaste, until Allah enriches them of His bounty.) This is a command from Allah to those who do not have the means to get married: they are to keep themselves chaste and avoid unlawful things, as the Prophet said:

«يَا مَعْشَرَ الشَّبَابِ مَنِ اسْتَطَاعَ مِنْكُمُ الْبَاءَةَ فَلْيَتَزَوَّجْ فَإِنَّهُ أَغَضُّ لِلْبَصَرِ وَأَحْصَنُ لِلْفَرْجِ، وَمَنْ لَمْ يَسْتَطِعْ فَعَلَيْهِ بِالصَّوْمِ فَإِنَّهُ لَهُ وِجَاءٌ»

(O young men, whoever among you can afford to get married, let him marry, for it is more effective in lowering the gaze and protecting the private parts. Whoever cannot do that, then let him fast, for it is a protection for him.) This Ayah is general in meaning, and the Ayah in Surat An-Nisa' is more specific, where Allah says:

﴿وَمَن لَّمْ يَسْتَطِعْ مِنكُمْ طَوْلاً أَن يَنكِحَ الْمُحْصَنَـتِ﴾

(And whoever of you have not the means wherewith to wed free believing women)until His statement;

﴿وَأَن تَصْبِرُواْ خَيْرٌ لَّكُمْ﴾

(but it is better for you that you practise self-restraint) ﴿4:25﴾ meaning, it is better for you to be patient and refrain from marrying slave-girl, because any child that is born will also be a slave.

﴿وَاللَّهُ غَفُورٌ رَّحِيمٌ﴾

(and Allah is Oft-Forgiving, Most Merciful) ﴿4:25﴾.

﴿وَلْيَسْتَعْفِفِ الَّذِينَ لاَ يَجِدُونَ نِكَاحاً﴾

(And let those who find not the financial means for marriage keep themselves chaste,) `Ikrimah said, "This refers to a man who sees a woman and it is as if he feels desire; if he has a wife then let him go to her and fulfill his desire with her, and if he does not have a wife, then let him ponder the kingdom of heaven and earth until Allah grants him means of livelihood.''