Hakim al-Umma Hadhrat Mawlana Shah Ashraf Ali Thanawi (may Allah have mercy on him) said;
‘Although, all the sins and attachments to other-than Allah are an obstacle in this path (sulook). Few indispensable ones are explained here.
First and foremost is opposition of the Sunna of Prophet, Allah’s blessings and peace be upon him (in beliefs, acts of worship, especially ikhlaq/morals, mutual dealings, financial transactions and all other activities).
Taleem ud Deen, page 131, Taj Co. edition. (An essential reading in Nisab e Tasawwuf)