In the Musnad of Imam Ahmad and the Sunans of At-Tirmidhi, An-Nasa'i and Ibn Majah it is recorded that Abu Hurayrah narrated that Allah's Messenger said:

«ثَلاَثةٌ لَا تُرَدُّ دَعْوَتُهُمْ: الْإمَامُ الْعَادِلُ، وَالصَّائِمُ حَتَّى يُفْطِرَ، وَدَعْوَةُ الْمَظْلُومِ، يَرْفَعُهَا اللهُ دُونَ الْغَمَامِ يَوْمَ الْقِيَامَةِ، وَتُفْتَحُ لَهَا أَبْوَابُ السَّمَاءِ، يَقُولُ: بِعِزَّتِي لَأَنْصُرَنَّكَ وَلَوْ بَعْدَ حِين»

(Three persons will not have their supplication rejected: the just ruler, the fasting person until breaking the fast, and the supplication of the oppressed person, for Allah raises it above the clouds on the Day of Resurrection, and the doors of heaven will be opened for it, and Allah says, `By My grace! I will certainly grant it for you, even if after a while.')

﴿أُحِلَّ لَكُمْ لَيْلَةَ الصِّيَامِ الرَّفَثُ إِلَى نِسَآئِكُمْ هُنَّ لِبَاسٌ لَّكُمْ وَأَنتُمْ لِبَاسٌ لَّهُنَّ عَلِمَ اللَّهُ أَنَّكُمْ كُنتُمْ تَخْتانُونَ أَنفُسَكُمْ فَتَابَ عَلَيْكُمْ وَعَفَا عَنكُمْ فَالـنَ بَـشِرُوهُنَّ وَابْتَغُواْ مَا كَتَبَ اللَّهُ لَكُمْ وَكُلُواْ وَاشْرَبُواْ حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ مِنَ الْفَجْرِ ثُمَّ أَتِمُّواْ الصِّيَامَ إِلَى الَّيْلِ وَلاَ تُبَـشِرُوهُنَّ وَأَنتُمْ عَـكِفُونَ فِي الْمَسَـجِدِ تِلْكَ حُدُودُ اللَّهِ فَلاَ تَقْرَبُوهَا كَذلِكَ يُبَيِّنُ اللَّهُ آيَاتِهِ لِلنَّاسِ لَعَلَّهُمْ يَتَّقُونَ ﴾

(187. It is made lawful for you to have sexual relations with your wives on the night of As-Siyam (fasting). They are Libas ﴿i.e., body-cover, or screen﴾ for you and you are Libas for them. Allah knows that you used to deceive yourselves, so He turned to you (accepted your repentance) and forgave you. So now have sexual relations with them and seek that which Allah has ordained for you (offspring), and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), then complete your fast till the nightfall. And do not have sexual relations with them (your wives) while you are in I`tikaf in the Masjids. These are the limits (set) by Allah, so approach them not. Thus does Allah make clear His Ayat to mankind that they may acquire Taqwa.)