When there is a valid Ikhtilaaf (difference of opinion) amongst authoritative scholars then it is permitted to follow it even if it is not from one’s own Madhab, this is called taking a Rukhsah.
In the Hanafi Madhab taking a Rukhsah is permitted with two conditions, firstly the opinion one takes cannot be a weak opinion and secondly it cannot lead to Talfeeq.
In Fath al-Qadeer Ibn Humaam said:
وَأَنَا لَا أَدْرِي مَا يَمْنَعُ هَذَا مِنْ النَّقْلِ أَوْ الْعَقْلِ وَكَوْنُ الْإِنْسَانِ يَتَّبِعُ مَا هُوَ أَخَفُّ عَلَى نَفْسِهِ مِنْ قَوْلِ مُجْتَهِدٍ مُسَوَّغٌ لَهُ الِاجْتِهَادُ مَا عَلِمْت مِنْ الشَّرْعِ ذَمَّهُ عَلَيْهِ، وَكَانَ – صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ – يُحِبُّ مَا خَفَّفَ عَنْ أُمَّتِهِ، وَاَللَّهُ سُبْحَانَهُ أَعْلَمُ بِالصَّوَابِ.
I do not know of anything that prevents a person from taking Rukhsah regularly neither from text (Quran and Hadith) nor Aql (rationality). Rather a person consistently following what is easy for him by taking from a qualified Mujtahid has no blame in Shari’ah. The Prophet used to love things that made it easier for his Ummah. And Allah (Most High) knows best.
In Radd al-Muhtaar Ibn Abideen said:
وَأَمَّا لَوْ صَلَّى يَوْمًا عَلَى مَذْهَبٍ وَأَرَادَ أَنْ يُصَلِّيَ يَوْمًا آخَرَ عَلَى غَيْرِهِ فَلَا يُمْنَعُ مِنْهُ
If a person were to pray according to a Madhab one day and wanted to pray according to another Madhab on another day, then there is nothing preventing him from doing so.
The first condition of Taking a Rukhsah is that it must be a sound opinion. Therefore it must be an opinion from one of the four Madhabs of Ahl Sunnah (or other recognised Mujtahids). And it must be a sound opinion in that Madhab, not a weak opinion. This must be ascertained from a reliable source and not merely hearsay.
The second condition is to avoid Talfeeq, which is to mix between the Madhaahib in such a way that the action will be invalid according to all. Ibn Abideen gives the example of a person who bleeds, then touches a woman and then prays Salah. His prayer is invalid in the Hanafi Madhab due to bleeding, and in the Shafi Madhab it is invalid due to touching a woman.
مِثَالُهُ: مُتَوَضِّئٌ سَالَ مِنْ بَدَنِهِ دَمٌ وَلَمَسَ امْرَأَةً ثُمَّ صَلَّى فَإِنَّ صِحَّةَ هَذِهِ الصَّلَاةِ مُلَفَّقَةٌ مِنْ مَذْهَبِ الشَّافِعِيِّ وَالْحَنَفِيِّ وَالتَّلْفِيقُ بَاطِلٌ فَصِحَّتُهُ مُنْتَفِيَةٌ. (رد المحتار)
Haskafi: The ruling regarding Talfeeq is that it is Batil (invalid) by agreement (Ijmaa). Ibn Abideen: An example of it is, a person made Wudu and then blood flowed from his body, then he touched a woman, and then prayed. The validity of this prayer would depend upon Talfeeq (mixing) between the Shafi and Hanafi Madhabs, and Talfeeq is Batil (invalid). Therefore the validity of the prayer is negated (i.e. the prayer is invalid). (Radd al-Muhtaar).
Imam Tahtawi said in his Haashiyah on Maraaqi al-Falaah:
ولا بأس بالتقليد كما في البحر والنهر لكن بشرط أن يلتزم جميع ما يوجبه ذلك الإمام لأن الحكم الملفق باطل بالإجماع
There is nothing wrong with following (another Madhab), as has been mentioned in al-Bahr and al- Nahr. However it is a condition that he observe everything that Imam obliges, because Talfeeq is Batil (invalid) by agreement (Ijmaa).